Sound reproducing apparatus



Feb. 16, 1943. H, BELAR 2,311,362

SOUND REPRODUCING APPARATUS Filed Dec. 29, 1959 son/v0 #540 6004 0 CONT/90L z/Mr Patented Feb. 16, 1943 SOUND ncraonncmc APPARATUS Herbert Belar, Philadelphia, Pa., assignor to Radio Corporation of America, a corporation of Delaware Application December 29, 1939, Serial No. 311,508

8 Claims.

This invention relates to sound reproducing apparatus and more particularly to an improved circuit arrangement for changing from standard sound reproduction to ush-pull or vice versa.

In push-pull reproduction, as heretofore manufactured and particularly in such apparatus designed for switching between push-pull reproduction and the standard or single reproduction, there has usually been a change in volume level between thedifferent types of reproduction and the amount of this change has in most instances been as great as 6 db. Such circuits also have required the use of resistors, condensers and other similar circuit elements within the soundhead which has been a'great disadvantage for the reason that these circuit elements became spattered with oil and accumulated dust thereby changing'their values and when repairs were necessary'on a soundhead for mechanical reasons} the delicatecircuit elements were readily damaged.

-In the improved circuit arrangement of the present invention, no resistors or capacitors are necessary in the soundhead and the balancing.

potentiometer which is particularly susceptible to the eifects of oil or dust is located in the control unit remote from the soundhead, the only critical element remaining in the soundhead being a transformer which is readily encased and shielded and the change-over switch which is of sufficiently rugged construction to give nomechanical trouble will cause no electrical trouble.

The purposes of this invention are accomplished by a novel circuit arrangement in which each of the photocells operates through the entire primary winding of the transformer whether they are connected in push-pull or parallel.

One object of the invention is to provide an improved photocell circuit.

Another object, is to provide an improved circuit for change-over from push-pull to standard or-vice 'versa in which;there is no change in volume level between the two types of reproduction..

Another object of the invention is an-improved sound reproducing circuit in which noxcritical circuit elements are located in the soundhead.

' ling as many soundheads as there may be in the Other and incidental objects of the invention In the several figures of the drawing which represent different aspects of the same apparatus, the same parts are represented by corresponding reference numerals and in the following description no distinction is made between the reference numerals as applied to the several figures.

The soundhead is generally indicated at l0, and the apparatus indicated within the outline is that which is located within the soundhead while the sound control ;unit which may be located'on the wall or at any other convenient position remote from the soundhead is generally indicated at ll and the apparatus pertaining to a single soundhead is indicated as being located therein. It should be understood that the sound control unit includes the apparatus for controlparticular installation, and it may also include all of the change-over equipment for the installation and such other control equipment as may be necessary. For each soundhead there will be provided in the sound control unit a duplicate of the apparatus indicated in Figure 1.

The photocells are indicated at I! and I3 and although two separate photocells are indicated, it should be understood that a single'photocell having a plurality of cathodes may be used, such as for example, thetype commercially known as the RCA No. 920. If such 'a photocell is used, the separate photocells referred to hereinafter should be construed as referring to the respec-* tive electrodes of the push-pull cell. The cell I2 is permanently connected to one end of the primary winding of the transformer l4 by the lead 22 and the cathode of the cell 12 is also permanently connected to the negative terminal of the B supply through the lead 23. The B supply as it is generally known but which may be otherwise defined as the high voltage direct current supply to the photocells preferably has a value of the order of volts if gas type photocells are used. If separate photocells are used, the voltage may be increased to such a value that 90 volts per cell is applied, and if vacuum type photocells are used, the voltage may be considerably in excess of this. When a push-pull gas lel.

type photocell is used, the voltage between any pair of photocells, and switching means for selectively connecting said photocells directly to said transformer either in push-pull or parallel with both of said photocells operating through the entire primary winding of the transformer.

4. In combination, an output transformer,'a

photocell directly connected to said transformer with its entire a-c output fed through the en.-

ode lead 24 is connected by the switch to the lead 25 placing it in parallel with the anode of the cell 13. The cathode lead 26 is connected to the cathode connection 23 of the cell l2. As shown in Fig. 2, the two cathodes are then connected to the negative terminal of the B supply while the two anodes are connected through the primary of the transformer M to the positive terminal on the potentiometer I B.

When the switch I5 is thrown into the upper position marked 3 in Fig. 1, the connections of the cell l3 are reversed so that it is then connected as shown in Fig. 3. In this case, thecathode of the cell l3 isconnected through the leads 26 and 25 to the anode of the cell l2 and thence through the primary of the transformer M to the mid-tap of the potentiometer I8. This mid-c. tap then serves as a negative terminal of the cell I3. The anode of the cell I3 is connected through the lead 24 to the lead 21 going to the positive terminal of the B supply, as shown in Fig. 3. When connected in this fashion the cells must .be balanced was to secure equal output from the two cells and this balancing is accoml plished by adjusting the position of the mid-tap of the potentiometer l6.

-As shown in Fig. 1,- the anode leads of the photocells are not connected, directly to the power supply but are supplied with current passed respectively by the'capacitors J9 and, 20.

tire primary of said transformer, a second photocell, and means for selectively connecting said second photocell either in parallel with said first ity reversed.

5. Sound-reproducing apparatus including in the sound-reproducing unit a pair of photocells, output coupling means and a switch for selectively connecting said photocells in either pushpull or in parallel relation to said coupling unit, and a control unit relatively remote from said sound-reproducing unit including balancing means for said photocells, feed resistors and capacitors bypassing said feed resistors.

6. In combination, an output device, a photo put circuit of said device, a. second photocell, andmeans selectively connecting said second photocell either in parallel with the first photocell or with one electrode opposite to that of ,the first two I through the resistor l1 and [8 which are by- 40 It will be apparent thatin the above described apparatus; the rect connection pf the photocell electrodes to t e transformer avoids the use of either capacitors or resistors'located adjacent photocells as shown in my Patent No. 2,l78,2l9, while at the same time retaining all of the ad vantages of the apparatus described in said pat- 2. In combination, an output transformer, a. pair of photocells, and switching means for selectively connecting both of said photocells directly across the entire primary winding of said transformer either in push-pull or in parallel.

3. Incombination, an output transformer, a

photocell connected to said outpfit device and the other electrode connected to a separate power supply.

'7. In combination, an optput device, a photocell connected, permanently directly across the entire input circuit of said device, a second photocell, means selectively connecting said second photocell either in parallel with the first photocell or with one electrodeopposite'to that of the first photocell connected to said output and the other .end terminal constituting said separate power supply terminal.

8. In combination, an output device, a photo- "cell connected permanently thereto, a second photocell, means selectively connecting saidsecond photocell either in parallel with the first photocell or with one electrode opposite to that of the first photocell connected to said output device and the other electrode connected to a separate power supply'terminal, a potentiometer for supplying current to said photocell, one end terminal and the mid-tap of the potentiometer be-- ing connected to the first photocell and the other end terminal constituting said separate power supply terminal, and resistors shunted by capacitors connecting said mld tap and the second end terminal to the photoeells.

HERBERT BELAR. 

